I understand that it means something like, "She is 6 feet tall, which is very tall." But this to me is such a bizarre and illogical way of conveying the message that it's distracting, and I cannot get myself to say it even if it comes to me. Anyone know the origin of this, or can breathe some reasoning into this phrase to justify its appreciation?

Well, certainly it's not illogical (even though logic often has little bearing on language): logically, it's just saying "If she's at least one inch tall, she's 6 feet tall". Since it's true that she's at least one inch tall, it's also true that she's 6 feet tall. Similarly "She's forty if she's a day", etc. It's just like "If I've told you once, I've told you a hundred times". Also It's not exclusively American English; I somehow seem to associate this construct with British English, though of course probably it's used with about the same frequency both sides of the pond.
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ShreevatsaRFeb 26 '11 at 4:25

1

This is a good question, English is full of idiomatic expressions which, if analysed, make little or no sense to the majority of us who use them. For example, we don't need to know what cat means in "no room to swing a cat". We know what the phrase as a whole stands for.
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RedGrittyBrickFeb 26 '11 at 10:58

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@ten: "At least" is implicit. And you're confusing statements with proofs. Whether a statement "If P, then Q" is a convincing proof is different from whether it's true or not. In logic, a statement "If P, then Q" is true unless P is true and Q is not. Thus, "If grass is green, then the sky is blue", "If pigs can fly, then the moon is made of cheese" and "If pigs can fly, then the sky is not blue" are all true statements in logic, even if the first part is unrelated to the second. In our case, "he's six feet tall" is the speaker's knowledge, it's not claimed to follow from "if he's an inch".
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ShreevatsaRFeb 26 '11 at 16:44

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I never said the statement is not true, I just said that it's not logical. And not in a technical sense but in common sense. The fact that the 2nd part is unrelated to the first is exactly why I think it's an illogical way to emphasize the first. Saying pigs can fly does not support the sky being blue. It seems like it would be more effective to use an actual supporting or emphatic comment rather than a meaningless one (e.g. "it's 100 degrees in the shade" rather than "it's 100 degrees and it's also at least 1 degree"). Maybe this is just easier to form in speech...
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tenfourFeb 26 '11 at 16:57

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@ten: See Robusto's answer and Matthew Burr's comment below for the explanation. (It's common in rhetoric to add a tautology or contradiction for emphasis; also consider "X is true or my name isn't…", even though one's name has no bearing on X. It's just saying "I'm as sure of X as I'm sure of my name" or "I'm as sure of his being six feet as I'm sure of his being at least one inch tall", etc.)
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ShreevatsaRFeb 26 '11 at 17:19

3 Answers
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Firstly, the meaning of "She's 6 feet tall if she's an inch" is not "She is 6 feet tall, which is very tall", but "She's definitely 6 feet tall" or "I'm very sure she's 6 feet tall". That is, it's an emphatic version of "She's [at least] 6 feet tall", with the emphasis being on the truth of the statement, not necessarily the great height. As such, in usage it typically (but not always) follows a statement made by someone else (borrowing from mgkrebbs):

The purpose here is not just entertainment, but rhetorical effect: it's saying "I'm as sure of her being 6 feet tall as I'm sure of her being at least an inch tall" — and since I am obviously very sure she's at least an inch tall, I'm also very sure she's 6 feet tall. It's equating her being 6 feet tall to her being an inch tall: saying (not necessarily with justification, but that's rhetoric for you) that if you deny that she's 6 feet tall, you must also deny she's at least an inch tall, which you obviously cannot do.

The use of "logic" in rhetoric

More generally, the use of such implication sentences in rhetoric seem (to me) to be of two kinds. If you want emphasise a statement X (say emphatically that X is true), you can pick either the form

"If T, then X" (= "X if T")
or

If not X, then F (= "X or F")

where T is some trivially true statement, and F is some trivially false statement. Examples of the "X if T" type include the ones you gave:

She's 6 feet tall if she's an inch.
It was a gallon of blood if it was a drop.
The baby was 10 pounds if it was an ounce.
She's forty if she's a day.
If I've told you once, I've told you a hundred times.
If I'm not hallucinating, I heard you come in at 3 am last night.

Examples of "If not X, then F" type include:

If he's a doctor, then I'm the Pope.1 (X = "he's not a doctor", F="I'm the Pope")
The prices will fall next week, or my name isn't [Name].
If he can hit a ball, then pigs can fly.
If that's pure gold, then the moon is made of cheese.

Not very good examples I'm afraid, but you get the idea.

[1: Entirely unrelated aside: There's an anecdote that once, when Bertrand Russell mentioned in a lecture that starting from a false premise you can prove anything, some smart alec said "1=2, prove you're the Pope". Russell immediately replied with "The Pope and I are two, therefore the Pope and I are one".]

This rhetorical device can of course be entertaining or humorous, as in Dorothy Parker's poem Comment:

Oh, life is a glorious cycle of song,
A medley of extemporanea!
And love is a thing that can never go wrong
And I am Marie of Roumania.

I disagree that the phrase is meant merely to be entertaining. What "if he's an inch" represents is, first of all, an example of the rhetorical device known as prolepsis, which in one of its meanings is the anticipation and addressing of objections to a premise before they may be introduced, in order to weaken opposing arguments.

By saying "She's six feet tall if she's an inch," the speaker is dismissing in advance all arguments that the subject's height may be less than the stated measure. He is in effect summing up the entire list of contradictory assertions ("Is she an inch tall? Is she two inches tall? Is she three ...?" and so on) with the obviously absurd notion that the subject might actually be so ridiculously small, and inviting the listener to laugh at such impossibilities.

For all its rhetorical resonance, however, this is also an example of the logical fallacy known as false dichotomy (similar to a "straw man" argument), in which two things are compared which are not in fact equivalent, so as to dismiss the one that is obviously false and therefore represent the alternative as truth. In actuality both may be false, and therein lies the fallacy. Fallacious arguments sound plausible, and actually constitute a kind of rhetoric (although a spurious one): today's "fair and balanced" news programs use them all the time to sway people who do not look deeply or critically into the "arguments" being made. While the statement "She's six feet tall if she's an inch" may be a bold and emphatic proclamation, it offers no real evidence. It indicates only that the speaker has a firm opinion and wishes to preempt further discussion on the matter.

All these points aside, the "X is [quality] ... if X is [inferior quality]" is a fairly standard and harmless formula in English, and if you want to sound like a native American (assuming you're not) it's one you should feel free to copy.

The phrase is perhaps better understood in a prototypical context. Imagine three acquaintances sitting around casually talking:

Man A: So, how tall is this fellow?

Man B: Oh, he's about five feet ten.
[inches]

Man C: Nah, he's six feet if he's an
inch.

The usage is colloquial and meant to be a more entertaining turn of phrase than a plain statement like "No. I think he is six feet tall." It probably started out being used in contradicting or contrasting situations like above, but it is also found standing alone. It doesn't mean "very tall" but more like "taller than was previously said".

It's an interesting idiom, though. It certainly can be entertaining, but it also has this emphatic feel to it. It turns a statement into an argument; no longer are we simply expressing opinions about something, we're now forced to deal with this argument: if he's an inch tall, then he's six feet tall. If I say, "no, he's five feet tall," then I'm also saying he's not even an inch tall. It's a silly argument, of course, but I wonder if that argumentative quality is what lends it that feeling of emphasis.
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Matthew BurrFeb 26 '11 at 8:04

@Matthew - I agree, it has an argumentative feel to it.
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mgkrebbsFeb 28 '11 at 8:17